Field of the Invention
The present invention relates, in general, to loudspeaker driver design and transducer apparatus and methods for improving the perceived listening performance and clarity of high-fidelity loudspeakers.
Discussion of Related Art
Prior art loudspeakers range from products that are relatively accurate reproducers of a sound field to lesser designs which provide marginal clarity and intelligibility. Loudspeaker design has evolved into a nuanced art, where designers evaluate sources of distortion generated within loudspeaker drivers as well as sources of distortion generated among or between drivers and other components in multi-way loudspeaker systems, like the 3-driver 2-way configuration now known as the D'Appolito “MTM” array, which was shown to reduce polar response asymmetries (see J. A. D'Appolito, Paper Number: 2000(F-2), AES Conference: 74th Convention (October 1983)) and has gained widespread acceptance in the high-fidelity loudspeaker marketplace.
Multi driver loudspeaker system designs have measurable practical performance limitations and there will always be a desire for greater fidelity or accuracy, louder playback levels (at specified distortion level maximums), lower distortion (at any playback level) and greater frequency response (at specified playback levels and distortion level maximums). Typical observations made by a layperson when first encountering high quality audio reproduction are that the sound is “clean” (meaning undistorted and accurately reproduced in rich detail) and “loud” (meaning that the sound pressure level of the playback approaches the sound pressure level (“SPL”) of the reproduced performance or event).
Loudspeaker systems typically have one set or array of transducers or drivers facing forward to provide the direct sound, and may optionally include a second identical set of transducers facing rearward (e.g., for Bipolar sound fields, where the rearward array is driven in phase to enhance the reflected sound field). The sound field consists of sound from the transducers which must be balanced to provide playback meeting the clarity requirements and reduced sound colorization is desirable.
Loudspeaker designers may evaluate performance of a design objectively by measuring acoustic performance and typically, loudspeakers are “voiced” either by ear, by measurements, or a combination of the two methods. The most common, and generally considered the most important, measurement is the on-axis free-field (anechoic) Sound Pressure Level (SPL) vs. frequency response which is measured with one or more microphones. Since humans do not listen as a microphone, humans interpret the complex sound field from a speaker and are sensitive to anomalies that produce distortions in the perceived sound.
Currently available high quality electro-acoustic cone diaphragm transducers, such as may be used in the loudspeaker systems described above, can create additional problems, depending on how they are configured (e.g., high frequency distortions caused by destructive interference within the transducer) and so the distortions and dynamic compression problems generated within each transducer must also be considered.
In conventional loudspeakers, efficiency requires a diaphragm which is both strong and light weight. Strength and light weight is typically achieved using a truncated cone shaped diaphragm with the minor diameter of the cone inside the transducer and the major diameter (flare or mouth) of the cone pointed out towards the front of the transducer. The cone shaped diaphragm may have straight or curved sides. The depth of the cone is such that at high frequencies the center of the cone may be ½ wavelength of sound deeper than the cone periphery, thereby causing the destructive interference described above. The destructive interference is frequency dependent, resulting in uneven frequency response, reduced efficiency, and audible distortion of the sound.
FIGS. 1B and 1C illustrate in an electrodynamic acoustic transducer 300 as previously disclosed and described in commonly owned U.S. patent application Ser. No. 13/162,296, the entire disclosure of which is incorporated by reference. That transducer or driver has a cone-shaped or frustoconical diaphragm attached at the periphery of its center opening to a voice coil, so that movement of the voice coil translates into axial reciprocating movement of the diaphragm. The voice coil is disposed on and is capable of moving along a cylindrical pole piece.
In the illustrated driver of FIGS. 1B and 1C, an annular spider 340 is attached at its outer periphery to an annular, planar spider support plateau 346 defined in frame 345. The inner periphery of the spider 340 is attached to the upper end of the bobbin or former carrying voice coil 315, below the diaphragm 310. In this way, the spider 340 provides elastic support for the voice coil 315, aligning and centering the voice coil 315 on the pole piece 320 in both radial and axial directions. Referring again to FIG. 1B, illustrating an upward facing driver 300, when at rest, spider 340 is substantially symmetrical or level, meaning that spider 340 is defined substantially within a single “at rest spider” plane which is substantially parallel with the planar surface of planar front plate 335, where the spider's inner peripheral attachment to the voice coil bobbin 315 and outer peripheral attachment to the frame 345 lie within that level spider plane. This level, symmetrical support for spider 340 is configured to provide a substantially equal elastic resistance to diaphragm excursion in either the push or pull excursion direction.
The spider 340 may be made from flexible material that can hold the voice coil 315 in place when the voice coil 315 is not driven by an electric current, and also allow the voice coil 315 to move up and down axially and (according to the models and specifications) symmetrically under influence of the electromotive push or pull force when the voice coil 315 is driven by an alternating electric current. Symmetrical response might be more properly characterized as the “ideal” or predicted response. The applicants have observed that actual loudspeaker suspension components don't exhibit this ideal symmetrical response, however. There are other loudspeaker components which are part of the suspension system for the driver's diaphragm, and those suspension components also fail to exhibit this ideal symmetrical response.
Applicants have discovered that when these prior art driver structures are combined into multi-driver systems, the resulting systems do not provide all that could be hoped for in the areas of low distortion and wide dynamic range. In order to obtain better performance in a cost effective way, there is a need for a loudspeaker configuration and method which overcomes the problems with the prior art and provides a cost effective improvement in sound quality for listeners using loudspeaker systems for sound projection or reproduction.